Always call credential->checkRequest() when creating an OAuth credential object. Fixes #929

From what I can see, various credential functions don't work if you don't call $credential->checkRequest() beforehand.

In User->getEmailAddress(), for example, $credential->getEmailFromOAuth() returns an empty string unless $credential->checkRequest() has been called previously.

It seems best to call checkRequest() immediately after creating $credential (or there are any cases where this would be expected to fail?)

See Issue #929 for repro instructions.